After cleavage of the membrane (M) protein of influenza A/WSN virus by using cyanogen bromide (CNBr), six peptide peaks representing approximate molecular weights of 6,000, 4,000, 2,2000, 1,600, 1,200, and 1,000 were resolved by gel filtration on BioGel P6. Analysis by thin-layer chromatography indicates that the first, second, fourth and fifth peaks contain single-peptide components, whereas the third and sixth peaks contain more than one peptide. By using Whatman CM52 ion-exchange chromatography in 5M urea, four peptides were resolved from the 3rd BioGel P6 peak. The amino acid composition of each of the purified peptides has been determined, and partial sequences were obtained for several peptides. Based on finding a blocked amino terminal residue, the 6,000-dalton fragment appears to contain the amino terminus of the M protein, whereas the carboxy terminal peptide was identified as a 2,000-dalton peptide.